Electro-optical displays that utilize electro-optical materials having an ability to transmit or block light dependent on whether the electro-optical material is in an electrically energized state or an electrically un-energized state have been used for many years.
Reflector plates have also been used for many years in reflective type electro-optical displays as a means of reflecting light back towards a transparent front plate of the display that has entered into the display through the transparent front plate and passed through an electro-optical material contained in the display and through a transparent back plate and impinged upon the reflector.
Reflectors used in the past in electro-optical displays typically feature smooth or roughened metallic surfaces or a diffusing means such as glass spheres imbedded in a hardened transparent medium depending upon the manner in which it is desired to reflect the light. Generally the type of reflection desired is dependent upon the chemical makeup of the electro-optical material contained in the electro-optical display through which the reflected light is transmitted. In some cases it may be desirable to reflect light uniformly in all directions by what are generally called "specular reflectors" of which examples are a mirror or highly polished metal surface. In other cases it may be desirous to reflect light in a selected direction by what are generally called "reflex reflectors".
An example of the use of a specular reflector in the form of a thin metal plate made from aluminum or silver is disclosed in U.S. Pat. No. 3,655,270. An example of utilizing a chromium plate electrode as a specular reflector in an optical display is disclosed in U.S. Pat. No. 3,756,694. Although such reflectors are proported to be of advantage for particular applications they involve the use of expensive silver and chromium and otherwise require the storage and handling of thin metal sheets that are subject to damage and present difficulties in adhering uniformly to the back plate of the optical display in which they are used as well as the undesirable aspect of polished aluminum in being unable to provide a wide reflecting angle.
An example of the use of a roughened metal surface or a composite reflector featuring a transparent diffusing front layer and a rear surface coated with a reflecting film of silver or aluminum is disclosed in U.S. Pat. Nos. 3,837,729 and 3,881,809. Although such reflectors are in common use today, they involve the problems of storage, handling and adherence associated with thin metal films as well as the expense of silver and roughening of metal surfaces and the complexities involved in providing a composite structure involving the combination of transparent diffusing material with a thin metal layer.
An example of the use of a powdered aluminum dispersed in a resin as a reflector in the form of a film coating on the back plate of an optical display is disclosed in U.S. Pat. No. 3,857,627. Such coating however involves the costly use of heavy basic metal material and the requirement of being used in conjunction with a polarizer film.
Heretofore it was believed that high reflectivity white background reflectors made, for example, from aluminum, ceramic or white paint were unsuitable for use in electro-optical displays in that they were only able to provide a fair contrast ratio between the electrically un-energized and energized state of the electro-optical material being used in the particular display. It has been discovered however that this theory is not correct at least in cases where the reflector incorporates an alkali earth metal salt and a binder in conjunction with a whitener or other colorant under such conditions as to enable the reflector to provide a high contrast ratio between the electrically energized and un-energized state of the electro-optical material.